250
FXUS63 KLSX 081121
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
621 AM CDT Sat Jun 8 2024

.KEY MESSAGES...

- Showers and isolated thunderstorms will continue across the
  region this morning, exiting the area around noon. Strong to
  severe storms are not anticipated this morning.

- Another round of thunderstorms is expected late this afternoon
  and evening, and these thunderstorms will be capable of isolated
  instances of damaging wind gusts and large hail. There remains
  uncertainly in where exactly these storms will form.

- Depending on where exactly storms this evening form and track
  tonight, locally heavy rainfall of up to 3-5" is possible. This
  may lead to a Flash Flood threat, primarily across portions of
  central and southeastern Missouri where a Flood Watch is now in
  effect.


&&

.SHORT TERM... (Through Sunday Night)
Issued at 247 AM CDT Sat Jun 8 2024

The latest regional radar imagery shows a decaying MCS moving
through portions of western Missouri. This MCS is outrunning the
unstable airmass it formed in, so the weakening trend of the system
is expected to continue as it approaches and then moves through the
CWA during the morning hours. While severe weather is not expected
with this MCS, it will be capable of locally heavy rainfall, with
ponding on roadways and full road-side ditches possible this
morning. However, its progressive nature is expected to limit a more
serious flood threat.

This MCS is expected to produce an outflow boundary that may linger
through the day somewhere across southern Missouri. South of this
outflow boundary, deterministic guidance and the 00z HREF shows
approximately 3,000 J/kg of SBCAPE among 30-40 kts of 0-6 km bulk
shear. As a subtle shortwave and surface low approach the CWA late
this afternoon and early this evening during peak heating,
convection is expected to develop along the boundary around 6pm.
Given the degree of instability and shear, multicell thunderstorms
with occasional supercellular structures are expected through the
evening, with the primary threats being damaging wind gusts and
large hail. Given the parallel orientation of the shear vectors
relative to the outflow boundary, upscale growth to line segments is
expected, which would lead to primarily a damaging wind threat. Low-
level shear and hodographs remain weak through the evening, so
barring a discrete supercell that can latch onto the boundary, the
threat for tornadoes looks very low at this time.

At this lead time, hi-res guidance still shows differing solutions
for the placement of the boundary and thusly where storms will
initiate. A more northern solution (e.g., the 00z HRRR) would have
convection forming across central and southeastern Missouri and
tracking southeast through the evening. A more southern solution
(e.g., the 00z NAM Nest) would have convection forming mostly across
south-central and far southeastern Missouri mostly outside of the
CWA.

Tonight, the low-level jet is expected to ramp up and interact with
the outflow boundary, causing convection to become increasingly
widespread and train along the boundary. This will pose a threat of
flooding where this occurs, as the 00z HREF has locally heavy
rainfall of 3-5" in locations across portions of south-central and
southeastern Missouri. Therefore, a Flood Watch has been issued for
portions of the area. If the boundary ends up being further south,
the threat of flooding would shift south as well.

Sunday, a cold front will push southward through the CWA as an upper-
level trough deepens over the eastern CONUS. This will push rain
chances southward through the morning, with much of the CWA seeing
dry conditions by the afternoon. Deep northerly to northwesterly
flow will begin advecting a cooler airmass into the CWA that will
combine with cloud cover in some areas to keep temperatures roughly
5-10 degrees below climatological normals Sunday afternoon into
Sunday night.

Elmore

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 247 AM CDT Sat Jun 8 2024

At the start of the period on Monday, deterministic and ensemble
guidance consensus has increased on that the axis of the upper-level
trough will swing through the region as deep northwesterly flow sets
up over the Middle Mississippi Valley and a shortwave remains
stalled over south-central Canada. As a result, high pressure at the
surface will spread into the Great Plains and Midwest, leading a
calm, cool day across the CWA. Confidence is high that this will be
the coolest day of the extended forecast period, with the 75th
percentile of ensemble guidance running about 5 degrees cooler than
climatological normals.

Upper-level ridging quickly builds into the Midwest as the trough
departs eastward on Tuesday per ensemble clusters. As a result, the
surface high will shift eastward, returning southerly low to mid-
level flow to the CWA. This will advect more seasonable air into the
CWA as we approach the middle of next week. While this ridge will
quickly transition eastward, guidance consensus is that another
ridge centered over the southwestern CONUS will strengthen and
expand, allowing for southerly flow to remain over the CWA.
Ensembles show the resulting temperatures climbing back above normal
(upper 80s - low 90s) by the end of the week, with narrow ensemble
spread increasing confidence in this return of more summer-like
weather.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 615 AM CDT Sat Jun 8 2024

A storm complex is currently moving eastward through the region.
Impacts from this complex will end shortly at KCOU, KJEF, and
KUIN, followed by KSUS, KSTL, and KCPS later this morning.
Moisture pooling ahead of a front approaching from the north will
lead to the development of low stratus that will make its way into
the area later this morning, mainly impacting KCOU, KJEF, and
KCOU. KSUS, KSTL, and KCPS are expected to be on the southern
periphery of this low stratus, so these three terminals may see
intermittent MVFR flight conditions this morning. Another round
of showers and thunderstorms is expected overnight, but the exact
location of these showers and thunderstorms is uncertain at this
time. The greatest chance for them is across central MO, impacting
KCOU and KJEF. Where storms track, multiple waves of them are
expected, leading to a potentially long period of VCTS or TS if
the track ends up being over local terminals. Shower and
thunderstorm chances do not wind down until right at or just
beyond the end of the current TAF period.

Elmore

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch from this evening through Sunday afternoon for Cole
     MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-Madison MO-
     Moniteau MO-Osage MO-Reynolds MO-Saint Francois MO-
     Washington MO.

IL...None.
&&

$$

WFO LSX